using java without bluej 5.0. 2 bluej projects a bluej project is stored in a directory on disk. a...

Using Java without BlueJ

5.0

2

BlueJ projects

• A BlueJ project is stored in a directory on disk.

• A BlueJ package is stored in several different files.

• Some files store the source code, some store the compiled code, some store additional information.

• BlueJ uses standard Java format for some files and adds some additional files with extra information.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

3

The BlueJ directory structure

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

UserInterfaceUserInterface

CalcEngineCalcEngine

CalculatorCalculator

project: calculator c:\bluej\calculator\Calculator.javaCalculator.classCalculator.ctxtCalcEngine.javaCalcEngine.classCalcEngine.ctxtpackage.bluejUserInterface.javaUserInterface.classUserInterface.ctxt

4

The BlueJ file structure

• package.bluej – the the package file. Contains information about classes in the package. One per package.

• *.java - standard Java source file (text). One per class.

• *.class - standard Java code file. One per class

• *.ctxt - BlueJ context file. Contains extra information for a class. One per class.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

5

Standard Java files

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• source files: *.javaJava source files contain the source code in readable form, as typed in by the programmer.

• source files: *.javaJava source files contain the source code in readable form, as typed in by the programmer.

• class files: *.classJava class files contain byte code (a machine readable version of the class). They are generated by the compiler from the source file.

• class files: *.classJava class files contain byte code (a machine readable version of the class). They are generated by the compiler from the source file.

6

The edit-compile-execute cycle

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

source file

011010

110101

010001

class file011010

110101

1001

10

1

0111

0110110

1

1

editorcompiler(javac)

virtual machine(java)

7

Editing

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• A file can be edited in any text editor– Notepad, emacs, jEdit, PFE, vi, ...

• Don't use Word: by default, Word does not save in text format

• Make sure to save with a .java filename before compiling!

8

Command line invocation

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• Compilation and execution of Java in JDK are done from a command line

• On Microsoft systems: DOS shell• On Unix: Unix shell• Must make sure that the

commands for compiler and runtime are in the command path.

9

Compiling

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• Name of the JDK compiler: javac• To invoke:javac <source name>

• compiles <source name> and all classes it depends on

• Example:cd C:\bluej\zuuljavac Game.java

10

Error messages

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

C:\bluej\zuul> javac Game.java

Game.java:22: ';' expected.

private Parser parser

^

1 error

C:\bluej\zuul>

The programmer has to open the file in the editor, find the line number, fix the error and recompile.

11

Execution

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• C:\bluej\zuul> java Game• “java” starts the Java virtual

machine.• The named class is loaded and

execution is started.• Other classes are loaded as

needed.• Only possible if class has been

compiled.

12

Problem: Execute what?

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• If we try:

C:\bluej\zuul> java GameException in thread "main" java.lang.NoSuchMethodError: main

• The problem: how does the system know which method to execute?

13

The main method

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• The answer: The java system always executes a method called main with a certain signature:

public static void main(String[] args){ ...}

• For this to work, such a method must exist!

14

The main method

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

• main must exist.• main must be public.• main must be static (class

method).• main must have a String[]

parameter.• Only main can be invoked.

15

Main method - example

• Consider placing in a separate class, containing just this.

• The main method should– create an object– call the first method

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

public static void main(String[] args){ Game game = new Game(); game.play();}